metadata exclusive uses

Prose defining mutual exclusive uses of <metadata> should use must keyword.

"Metadata information may be expressed by a metadata element in one of two (non-mutually exclusive) manners: (1) by specifying one or more metadata attributes on the metadata element, and (2) by specifying one or more child elements in the metadata element that express metadata. Both types of metadata are referred to in this document as metadata items."

By using "may" in this normative prose, a reader may fail to notice that this mutual exclusive usage definition has the intended force of "must" in the sense that a given metadata element MUST NOT express metadata attributes and metadata element child elements.

To make this clear, the prose should be modified to read as follows:

"A metadata element must not express metadata information by both (1) specifying one or more metadata attributes and (2) specifying one or more metadata child elements. Both types of metadata are referred to in this document as metadata items."

"Note: In other words, a metadata element may specify, in a mutually exclusive manner, either (1) metadata attributes or (2) metadata child elements, but it may not specify both forms of metadata information together."

Note also that this constraint is not expressible in either XSD or RNC schemas, and must be verified during a semantics verification phase independently from schema validity assessment.